This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. P2001-275700, filed on Sep. 11, 2001, the entire content of which is incorporated herein by reference.
The present invention relates to an X-ray flat panel detector which is usually used for an X-ray diagnosis apparatus and reduces a noise caused in an X-ray image generated in the detector. The present invention further relates to the X-ray diagnosis apparatus and to a method of improving the X-ray image in both of which a noise is reduced.
An X-ray diagnosis apparatus has been used for a medical diagnosis. An X-ray diagnosis image is usually produced in a well-known manner, for example, shown as follows. X-rays enter into a patient body and a two dimensional X-ray image is produced by the difference of X-ray absorption in each tissue of a human body. The produced two-dimensional X-ray image is detected by an X-ray detector through several processes and converted to an electric signal. As a result, a visible X-ray image is obtained for a diagnosis. Recently, an X-ray flat panel detector (hereinafter referred to as FPD) has been introduced in the market as a new type of the X-ray detector.
The FPD has an array comprising a plurality of pixels in a form of a matrix. Each pixel comprises an X-ray detecting portion which converts entered X-ray signals to charge signals, a capacitor which accumulates the detected charges, and a semiconductor switch which selects to read out the detected charges from the capacitor.
When the detected charges are read out, the semiconductor switches provided in the same pixel row are switched on and off sequentially pixel row by row of the array (matrix) and the charge signals are taken out. The taken out charge signals are converted to voltage signals and amplified in amplifying portions provided in each pixel column, respectively, and are taken out as digital signals.
The digital signals usually include an offset noise (undesired offset amount) and a line artifact noise (which is a high frequency component that is stable in a row direction of the matrix and is fluctuant or fluctuates in a column direction of the matrix; the details of which are given below) as well as desirable signals converted on the basis of the entered X-ray signals. In addition, there are also a fluctuation in conversion efficiency by the X-ray detecting portions and a fluctuation in amplifying efficiency by the amplifying portions. Therefore, in order to obtain only the desirable signals based on the entered X-ray signals, it may be necessary to correct such undesired noises and fluctuations.
Practically, it is usually possible to obtain correction data for each of the offset noise, the fluctuation in conversion efficiency, and the fluctuation in amplifying efficiency in advance of an actual examination by an X-ray diagnosis apparatus. Accordingly, the obtained correction data are used for image corrections afterwards.
When, however, it comes to the line artifact noise, correction data is not typically available in advance. In general, the line artifact noise is thought to be originating from a temporal fluctuation in a signal (gate signal) for switching on/off the semiconductor switches in the same pixel row, row by row. A line (gate line) for conveying a signal for switching on/off the semiconductor switches provided in the same pixel row is provided to be insulated from a line (signal line) for transferring the charge signals accumulated in each pixel to the amplifying portion. Each signal line is commonly used for pixels in each pixel column. However, the insulation may often be practically imperfect, and this may cause an apparent stray capacitance to exist in each crossing portion of between the gate line and the signal line. Accordingly, even when it is not time to switch on the semiconductor switches provided in a pixel row, the apparent stray capacitance related to the pixel row is read out, responsive to the fluctuation of the gate signal as a noise included in the gate signal. This results in that such an apparent stray capacitance comes to be laced in the charge signals read out from the capacitor provided in each pixel. Since a noise included in the gate signal is usually different in each actual examination, the line artifact noise may usually be different in each actual examination. This is why the correction data for the line artifact noise is not typically available in advance of an actual examination by an X-ray diagnosis apparatus.
For such a line artifact noise, one solution has been presented as follows. Since the line artifact noise is temporally random and not available for preparing correction data in advance, one or more of columns of X-ray blind pixels (hereinafter referred to as dark lines) are prepared at one or both end(s) of the FPD. Information originated from a gate signal may be sampled from the dark lines during an actual examination and the sampled information is used to prepare one dimensional correction data for each pixel row. The prepared correction data is applied to a line artifact noise in real time during the actual examination.
The above method of correcting a line artifact noise may have some effect. Since, however, the above method is of a correction using one dimensional correction data for each pixel row, it still cannot solve a problem of a fluctuation of a line artifact noise in each pixel column, that is, the difference of a noise size in each pixel column even in the same pixel row.
As for causes of the fluctuation of the line artifact noise in each pixel column, there may be mentioned the following three factors: (1) accuracy of correcting the fluctuation in amplifying efficiency of the amplifying portion in each pixel column; (2) the difference of the apparent stray capacitance in each crossing portion of between the gate line and the signal line; and (3) intergradations of the line artifact noise in response with transferring distances due to a resistance or impedance component of the gate line. For example, taking the factor (3) into consideration, the larger the FPD is, the worse its correction accuracy seems to become.
Consequently, with the prior art method of correcting the line artifact noise, a preferable correction was not performed on the line artifact noise (which is temporally random) which fluctuates in each pixel column. This caused an X-ray diagnosis image to have noise remaining due to the failure of clearing the line artifact noise and noise remaining as a result of exceeding correction of the line artifact noise (collectively hereinafter referred to as remaining line artifact noise). After all, it may have resulted in a problem in an X-ray image diagnosis.
Further, the prior art correction data for the fluctuation in conversion efficiency of the X-ray detecting portions is typically obtained by calculating a fluctuation in each pixel, in advance, by using an X-ray image obtained through the pixels with X-rays evenly entered into the pixels without a subject and an image without X-rays entrance. Such correction data, however, includes a temporally unchanged artifact in a form of lines. This is because the correction data is prepared on the basis of the X-ray image including a remaining line artifact noise. Therefore if such correction data are used for correcting the fluctuation in conversion efficiency of an X-ray image, the corrected X-ray image unintentionally has a similar temporally unchanged artifact in a form of lines.
According to a first aspect of the present invention, there is provided a flat panel detector, having a plurality of pixels in a matrix, for detecting an X-ray image, which comprises first filtering means for filtering out a high frequency in a column direction of a first X-ray image obtained through at least a portion of the pixels from the first X-ray image, first subtracting means for subtracting the image obtained by the first filtering means from the first X-ray image, second filtering means for filtering out a high frequency in a row direction of the image obtained by the first subtracting means from the image obtained by the first subtracting means, and second subtracting means for subtracting the image obtained by the second filtering means from the first X-ray image.
According to a second aspect of the present invention, there is provided a flat panel detector, having a plurality of pixels in a matrix, for detecting an X-ray image, which comprises first filtering means for filtering out, a high frequency in a column direction of a first X-ray image obtained through the pixels with an X-ray evenly entered into the pixels without a subject, from the first X-ray image, first subtracting means for subtracting the image obtained by the first filtering means from the first X-ray image, second filtering means for filtering out a high frequency in a row direction of the image obtained by the first subtracting means from the image obtained by the first subtracting means, and second subtracting means for subtracting the image obtained by the second filtering means from the first X-ray image.
According to a third aspect of the present invention, there is provided an X-ray diagnosis apparatus for converting an X-ray into an electric signal, which comprises extracting means for extracting a noise element, which is fluctuant in a column direction, included in a first X-ray image obtained on the basis of the electric signal, and subtracting means for subtracting the noise element extracted by the extracting means from the first X-ray image.
According to a fourth aspect of the present invention, there is provided a method of correcting an X-ray image detected in a flat panel detector having a plurality of pixels in a matrix, which comprises a first step of filtering out a high frequency in a column direction of the X-ray image from the X-ray image, a second step of subtracting the image obtained in the first step from the X-ray image, a third step of filtering out a high frequency in a row direction of the image obtained in the second step from the image obtained in the second step, and a fourth step of subtracting the image obtained in the third step from the X-ray image.
According to a fifth aspect of the present invention, there is provided a computer program product on which is stored a computer program for reducing a noise appearing in an X-ray image, the computer program having instructions, which when executed perform steps comprising filtering out a high frequency in a column direction of the X-ray image from the X-ray image, subtracting the image obtained by the column direction filtering step from the X-ray image, filtering out a high frequency in a row direction of the image obtained by the subtracting step from the image obtained by the subtracting, and subtracting the image obtained by the row direction filtering step from the X-ray image.
According to a sixth aspect of the present invention, there is provided a method of correcting an X-ray image obtained from an X-ray diagnosis apparatus, which comprises a step of extracting a noise element, which is stable in a row direction and fluctuant in a column direction, included in the X-ray image, and a step of subtracting the noise element extracted by the extracting step from the X-ray image.